Iron-core transformer construction



May 20, 1941. R E 2,242,359

IRON-CORE TRANSFORMER CONSTRUCTION Filed Sept so, 1938 2 Sheets-Sheet 1 [mu/aim.

WITNESSES: INVENTOR ATTORNEY May 20, 1941. LEE 2,242,359

IRON- CORE TRANSFORMER CONSTRUCTION Filed Sept. 30, 1938 2 Sheets-Sheet 2 WITNESSES: 'INVENTOR 4%71 K? M-- Reuben Lee.

g 6 BY Patented May 20, 1941 IRON- CORE TRANSFORMER CONSTRUCTION Reuben Lee, Catonsville, Md., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application September 30, 1938, Serial No. 232,626

10 Claims.

My invention relates to electrical transformers and particularly to transformers for use in audio frequency and radio frequency circuits.

One object of my invention is to provide a transformer for the above-mentioned fields of use which can be insulated with ceramic insulating materials and in which the ceramic insulators shall be of such a form that they can be readily and cheaply produced by methods well known in the ceramic arts.

Another object of my invention is to produce a transformer in which the leakage inductance and electrostatic capacity between the primary and secondary windings are reduced to very low value.

Another object of my invention is to produce a transformer in which core losses at high frequencies are minimized.

Another object of my invention is to produce a transformer which can be constructed in the factory by methods which are economical and which do not require expensive special apparatus or highly specialized labor.

A still further object of my invention is to produce a transformer which is rugged in structure and which employs insulations having low dielectric losses and capable of withstanding relatively high temperatures.

Other objects of my invention will become apparent upon reading the following description in connection with the drawings in which:

Figure 1 is a sectional View, taken perpendicular to the axis of the principal magnetic core;

Figs. 2 and 3 are, respectively, a side elevation and an end view of one of the ceramic insulators employed in the construction of the transformer appearing in Fig. 1;

Fig. 4 is a detailed view in elevation of an end plate of ceramic material used in the transformer of Fig. 1;

Fig. 5 is a detailed view of an alternative type of insulator which may replace the insulators shown in Figs. 2 and 3;

Fig. 6 is a diagram used in stating dimensions of the magnetic core of my transformer;

Fig. '7 is a sectional view taken along the line VII-VII of Fig. 8;

Fig, 8 is an end elevation of a set of windings for an alternative modification of my invention;

Fig. 9 is a circuit diagram illustrating a use of the last-mentioned modification, and

Fig. 10 is a view of the last-mentioned modification of my invention assembled with an electrical discharge tube.

It was formerly impracticable to employ iron core transformers in radio frequency circuits because of the large core losses at such high frequencies. Air core transformers were employed, but these required the use of rather complicated networks of tuned circuits in order tokeep the size and cost of the transformers within a reasonable figure. I have found that by employment of ferromagnetic cores of certain structure and alloy composition, it is possible to use transformers with ferromagnetic cores even at frequencies as high as 600 kilocycles and above, thereby avoiding the use of the complicated and expensive tuned networks for the output circuit of radio transmitter tubes.

However, these transformers must differ in design and structure very radically from the ferromagnetic-core transformers which are employed in ordinary power circuits, and it is the object of the present application to disclose several structures which I have found suitable for my above-mentioned purposes. For one thing, to keep core losses down to a reasonable figure, a relatively low magnetic flux density must be employed in the ferromagnetic core. For another thing, the leakage inductance and electrostatic capacity between the primary and secondary windings must be reduced to relatively low values. The foregoing objectives are effected by employing cores of relatively large cross-section in conjunction with windings having a relatively small number of turns. Also, the windings are interleaved to reduce the magnetic leakage, and this interleaving is carried out to a greater degree than in the case of power transformers operating on commercial frequencies.

For another thing, the electrostatic capacity between the windings, and from winding to ground, is reduced by increasing the distance separating the primary and secondary windings from each other and from the transformer core and casing structure, and the presence of solid dielectric material intervening between the primary and secondary windings, and between each winding and the remainder of the transformer structure, has been reduced to the minimum. Likewise, inorganic insulations have been employed, as they are found to be preferable to the more usual organic insulations at the frequencies in which my transformer is expected to operate.

With the foregoing principles in mind, attention is directed to Fig. 1 of the drawings which shows a horizontal cross-section taken through the ferromagnetic core and winding structure of one embodiment of my invention. The ferromagnetic core I may be of the shell type and may consist of laminated sheets approximately .005 to .014 inch thick consisting of a suitable ferromagnetic alloy such as high percentage sili- Y con steel. This core I is intended to operate at a flux density of approximately 16 gausses when the frequency of alternation is 600,000 per second. Surrounding the central leg of the core I is an insulating sleeve 2 which may be organic material approximately T16 inch thick. Each side of sleeve 2 is faced with a plate 3 of insulating material which may be Mycalex, mica, porcelain, etc., and on which is superposed an insulating member 4 which is preferably made from the inorganic insulator Isolantite, but may alternatively be of the inorganic insulator Steatite, Mycalex, or of porcelain, or even of glass, and which is shown in detail in Figs. 2 and 3.

As wifll be seen from Fig. 2, the insulator 4 has, between the flanges at its two ends, a depressed surface forming a rounded contour. A winding 5 which may be of any suitable metal has an insulating coating and in cross-section a number of turns depending upon the current and voltage of the load which is being energized. This winding may be employed either as a primary or a secondary winding. The successive turns should preferably be adjacent to one another, although the provision of grooves in the insulating material for maintaining a slight separation between successive turns is within the scope of my invention. Once applied, the turns may be cemented to each other or to the insulators 4 by any suitable cement, such as insulating varnish, and their ends may be connected to external circuits through binding posts supported in a pair of insulating end plates for the transformer structure which will be referred to in more detail later on.

After the aforesaid winding is in position, the space between the flanges on the insulators 4 may be closed in, if. desired, by side walls in the form of plates 6 of insulating material which I prefer to be of Isolantite, Steatite, porclain, Mycalex, etc.

The length of the insulators 4 along the winding is preferably made comparatively short so as to leave an open space between the greater part of the winding 5 and the insulating plates 3.

superposed at each corner of the box-like enclosure previously described is an insulating angle-member 8 which may be of the same material as the insulator 4 and may, like it, be provided with flanges at each end and a depressed portion suitable for receiving the winding between these flanges. The insulator 8 is so similar in general structure to the insulator 4 that I have shown no detailed drawing of it, believing that the minor alterations in its shape indicated in Fig. 1 are sufficient to disclose to a man skilled in the art the only differences between its structure and that of the insulator t. Again, as in the case of the insulator 4, the length of the insulator 8 parallel to each side of the core I is made relatively short so as to leave an open space between the side of each insulator 8 and the insulator 8 at the next corner of. the core. A winding 9, which may be of any suitable size and number of turns of insulator conductor, is Wound tightly over the four insulators 3 as they are placed in position and may be cemented thereto by. any suitable cement.

The structure sofar described having been assembled, enclosing walls H, which may be plates of such an insulating material as Mycalex, Isolantite, Steatite, porcelain, etc., may be installed in position resting on the flanges of the insulators 8, thereby forming a box-like enclosure for the entire winding structure. End plates of the form shown in Fig. 4, which may be of any suitable insulating material such as Isolantite, Steatite, Mycalex, etc., may then be placed in position to close the top and bottom ends of this box-like structure. These end plates may each support two binding posts l2, and the respective ends of the two windings 5 and 9 may be connected to the inner ends of such binding posts.

The structure comprising an the elements so far described, except the magnetic core I, may, if desired, be assembled separately therefrom and a winding I2 of tape be wrapped about opposite legs of this structure to hold the various portions thereof securely in place. The insulating members 3, 4, 8 and H may also be cemented to each other, if desired, by a suitable cement such as Insalute. The assembled structure may then be placed upon the ferromagnetic core I in accordance with methods of transformer construction well known in the art.

It will be seen that the insulators 4 and 8 are sufiicient to provide insulating support for the windings 5 and 9, and it is, accordingly, within the scope of my invention to omit any or all of the insulating casings 2, 6 and II.

Instead of supporting the windings 5 and 9 each upon four separate insulators like 4 and 8, the insulators 4 may be consolidated into a single structure by providing struts connecting them at each of their flanged ends. Each end of the insulating structure substituted for insulators 4 will then be of the form shown in Fig. 5. A similar substitution may be made of a single member of the general outline shown in Fig. 5 for the four separate insulators 8. It is also within the scope of my invention that there may be substituted for each set of four insulators 8 and d a cylindrical structure such as would be formed by extending them toward each other until they met and there was no space intervening between their adjacent edges. The end view of such a cylindrical structure would then be represented also by Fig. 5.

To illustrate one specific embodiment of my invention, the magnetic core I may consist of laminations of silicon steel alloy .005 inch thick and of such number and dimensions as to form a central core 1% inches by 4 inches. The length of this core perpendicular to the section shown in Fig. 1 may be 3 inches and the dimensions indicated for this core by the letters in Fig. 6 be as follows:

A=1% inches 3:2 4 inches The casing 2 may be of Micarta inch thick, and the plates 3 may be of Mycalex 1 5 inch thick by 1% inches wide; those normal to the core laminations in Fig. l are 31% inches long while those parallel to the core laminations are 1%; inches long.

The insulators 4 may have an overall length of 1%, inches, their flanges being inch thick by inch high at their ends; the distance from the center of the angle to the side-edge which is perpendicular to the plane of Fig. i may be about inch and the thickness of each side leg be about inch.

The winding 5 may be of number 13 B. & S. gauge copper wire insulated with single-cotton covered enamel. The side plates 6 may be of Mycalex about 1% inch thick by 1%; inches wide. The two plates 6 lying normal to the core laminations in Fig. 1 may be 4%}; inches long, and the pair lying parallel to the core laminations be 21 inches long. The insulators 3 may have an overall length of 1% inches and the flanges at each end be inch thick by A; inch high at the sides of the angle. The thickness of the intermediate portion of insulators 8 upon which the winding is superposed may be about inch, and the length of each limb of the L-shaped inner corner of insulators 8 may be about inch.

The winding 3 may be of copper conductor of #28 B. & S. gauge having single cotton covered enamel insulation and comprising 100 turns. The insulating-casing plates H may be of Mycalex approximately inch thick, 2% inches wide and 5% inches long.

The transformer having the dimensions just described is adapted to operate on an alternatingcurrent source having a frequency of 100 to 600 kilocycles with a primary voltage of 1350 volts, a secondary voltage of 150 volts, a power input of 250 watts and a power output of about 200 watts.

Referring to the modification of my invention illustrated in Figs. 6 to 9, inclusive, the primary and secondary windings are each formed as a plurality of what may be termed pancake coils mounted in spaced relation with each other on a tube support of insulating material. Thus, a tube 20, which may be of fiber or other suitable insulating material, supports eight spacing rods 2| which may be of Mycalex or other suitable insulating material. At each end and in the middle of the supporting structure thus formed are positioned three rings 22, which may be of material similar to the tube 20, and on each of these rings is wound a coil 23, 24, 25, the three coils being adapted to be connected in series with each other to act as a secondary winding for the output energy of a radio frequency amplifier or the like. Midway of each of the two spaces separating the above-mentioned three coils is positioned a ring 22' similar to the rings 22 previously mentioned and upon which is wound a pair of coils 26 and 21 which are adapted to constitute two halves of a primary winding cooperating with the aforesaid secondary winding. To maintain the spacing unaltered between the above-mentioned primary and secondary windings, a collar 28, which may be of the same material as the tube 20, is positioned between each pair of adjacent windings. The respective coils 23 to 21 may each be wound on its supporting ring by methods well known in the art to form a unitary pancake coil and may then be impregnated with insulating gum, such as Varnish #3395. After solidification of the latter, the coil is sufficiently rigid to maintain its form. Leads from the radially-inner ends of the windings 26 and 21 may pass through holes in the collar 28 into the space between the latter and the tube 20, and may there be joined and led out for connection to an external circuit. The radially-outward ends of the windings 26 and 21 are, respectively, provided with flexible leads for connection to external circuits in a manner to be described more fully below in connection with Fig. 9. The radiallyinner and the radially-outer ends of the windings 23, 24 and 25 are likewise brought down into the annular space between the tube 20 and the collars 28 and brought out for connection to external circuits; or alternatively, they may be brought by the nearest route to a terminal board.

As indicated in Fig. 9, the inner ends of the windings 2B and 21 are adapted to be connected to the positive terminal of the source of direct current supplying the plate circuits of a pushpull amplifier, and this terminal is grounded for radio frequency currents through an appropriate condenser 29 in the amplifier circuit. The radially-outer ends of the two windings 26 and 21 are respectively connected to the plate terminals of two tubes .connected to constitute a push-pull amplifier, as indicated in Fig. 9. The windings 23, 24, 25 may be connected either in series or in parallel with each other as desired to constitute the secondary circuit for transmitting the output of the aforesaid amplifier, and this secondary circuit will, in general, be grounded. As the secondary circuit, in general, generates a relatively low radio frequency potential, it follows that all parts of it are at nearly ground potential, and it will, therefore, be seen that all of the leads passing through the annular space between tube 20 and collars 28 are at nearly the same, i. e. at ground, potential. On the other hand, a radio frequency voltage of substantial magnitude will, in general, be induced in the windings 26 and 21 so that the radially-outward end of each of these windings differs substantially from ground potential.

It will be seen that with the winding arrangement and structure above described, the leads from the outer ends of the windings 26 and 21, which are expected to oscillate at substantial potentials relative to ground, and hence to the other windings and the transformer core, are separated both from the secondary windings and from other portions of the electrical system at ground potential by relatively wide air spaces. Thus, the outer end of winding 26 is spaced by a considerable distance from the nearest points on secondary windings 23 and 24 and is also spaced by a considerable distance from its own radially-inner end and from the magnetic core of the transformer which is intended to pass through the interior of the tube 20. This wide spacing not only insures effective insulation of the high potential ends of windings 26 and 21, but also reduces to a minimum the capacity to ground, and to the grounded secondary windings, of said radially-outward end and the leads passing therefrom to the plate electrodes of the amplifier tubes.

As was pointed out in connection with Fig. 1 of my invention, the preservation of wide spacing between the magnetic core and those portions of the windings which are to oscillate at substantial potentials is highly desirable, and it will be noted that the modification of my invention shown in Fig. 7 is an alternative arrangement to that in Fig. 1 for maintaining such wide spacing.

The coil structure shown in Figs. 7 and 8 having been thus assembled, it is provided with a magnetic core of the shell type, the central leg of the core passing through and nearly filling the interior of the insulating tube 20, and the laminations preferably being in planes parallel to that of Fig. 7.

Fig. 10 shows how the assembled windings and core will appear when connected in a preferred way with the tubes of the push-pull oscillator. Thus, a magnetic core 3| has its laminations extending in vertical planes perpendicular to the plane of Fig. 10. The windings 23 to 21 appear in side elevation and the leads 32 extend from the radially-outward end of winding 26 to the anode 33 of one tube 34 of a push-pull oscillator. A similar lead, which does not appear in this figure because it is positioned directly behind lead 32, extends from the radially-outward end of winding 21 to the plate terminal of the other tube of the push-pull oscillator which is positioned directly behind the tube 34, as shown in Fig. 10. The laminations of the core 3| are held rigidly in position by a pair of end-frames 35 of a form well known in the art. The windows or openings through which windings 23 to 2'! pass through the laminated core 3| are made of such dimensions that the outside legs of the core are spaced away from the peripheral edge of the windings 26 and 21 by a distance great enough to insure that the electrostatic capacity between said peripheral edge and the aforesaid outer limbs of the core is kept to a low value as is that between the high potential peripheral turn of the winding 25 and the central leg of the magnetic core.

In order to insure that the respective windings 23 to 2'! always maintain their original spacings, plates 30 of mica or other suitable insulating material may be provided with three notches dimensioned to fit over the outer edges of windings 24, 20 and 21 and may be clamped between the core and the end-frames 35 when the transformer is assembled. In this way, it may be insured that the respective windings 24 to 21 remain fixed in relative position throughout the subsequent use of the transformer. In many cases, however, the use of the members 20 will be found to be unnecessary.

As a specific illustration of the transformer illustrated in Figs. '7 to 10, the insulating tube 20 may be of fiber 2% inches long and having walls reth of an inch thick, the outside dimensions of its cross-section being 2 inches by 4% inches. The spacers 2i may be of Mycalex 2% inches by ths of an inch by Agth of an inch. The rings 22 may be fiends of an inch long, have walls feth of an inch thick, and have outside dimensions of 3 inches by 5% inches. The collars 28 may be of fiber g inds of an inch long and having walls Teth of an inch thick. The outside dimensions of their cross section may be three inches by 5% inches.

The secondary windings 23, 24 and 25 may be wound from No. 13 double-cotton-covered enameled wire of No. 13 Brown Sz Sharpe gauge, each being formed with a total of 15 turns constituting a single spiral. The coil will, accordingly, have a thickness in the direction of its axis of about {ends of an inch.

The windings 26 and 21 should each be wound from 95 turns of copper strip double-cotton-covered 0.05 inch wide by 0.004 inch thick. When the five coils 23 to 2! have thus been wound and mounted as earlier described on the insulating structure 2!, 22, 28, the assembly is provided with a magnetic core comprising laminations of high percentage silicon steel about 0.005 inch thick, enough such laminations being used so that, when compressed between end-frames, they have an aggregative thickness of 3% inches. The central leg passing through the central holes in the coils 123 to 23 will then be approximately 3% inches by 2%; inches and will have a length of 2 inches. The lettered dimensions in Fig. 6 which show the form of the core may then have the following dimensions:

A= /4 inches B=2%; inches C=2 /4 inches, and D=l inches.

he above-described transformer is adapted to have its primary terminals connected to the anodes of a pair of high-vacuum radio tubes of type 805 having a rating of 300 watts radio frequency output and employing a plate voltage of 1250. The secondary windings 23, 24 and 25, connected in series as above described, Will operate with a maximum output current of about 1.0 ampere, and this transformer is adapted to operate at a frequency of about 600,000 cycles per second.

While, in accordance with the patent statutes, I have described several specific embodiments of my invention, it will be recognized that the principles thereof are of broader application, which will be evident to those skilled in the art.

I claim as my invention:

1. A transformer structure comprising a rectangular cylindrical magnetic core, a sheath of inorganic insulation having an L-section fitting each corner of said core and leaving an open space between adjacent edges of the different sheaths, a Winding supported upon said sheaths and surrounding said core, a second sheath of inorganic insulation of L-section for each corner of said core superposed above the first-mentioned sheaths, and a winding supported upon said second sheath and enclosing said core.

2. A transformer structure comprisinga rectangular cylindrical magnetic core, a sheath of inorganic insulation having an L-section fitting each corner of said core and leaving an open space between adjacent edges of the different sheaths, a winding supported upon said sheaths and surrounding said core, a second sheath of inorganic insulation of L-section for each corner of said core superposed above the first-mentioned sheaths, a winding supported upon said second sheath and enclosing said core, and end plates of insulation enclosing said sheaths and said windings.

3. A transformer comprising a cylindrical core of ferromagnetic material having a rectangular cross-section, an insulating facing for each side of said cylindrical core, a. sheath of inorganic insulation of L-section fitting each corner of said core and leaving an open space between ad- J'acent edges of the sheaths of different corners of said core, each said sheath having a flange at each of its ends, a winding supported on a portion of said sheaths between said flanges and enclosing said core, a second sheath of inorganic insulation for each corner of said core superposed above each first-mentioned sheath and leaving an open space between adjacent edges of the second sheaths for different corners, said second sheaths each having a flange at each end, a second winding supported on the portions of said second sheaths between their flanges and surrounding said core, and end plates of insulation enclosing said sheaths and said windings.

4. A transformer comprising a cylindrical core of ferromagnetic material having a rectangular cross-section, an insulating facing for each side of said cylindrical core, a sheath of inorganic insulation of L-section fitting each corner of said core and leaving an open space between adjacent edges of the sheaths of different corners of said core, each said sheath having a. flange at each of its ends, a Winding supported on a portion of said sheaths between said flanges and enclosing said core, a cylindrical partition encircling said winding, a second sheath of inorganic insulation for each corner of said core superposed above each first-mentioned sheath and leaving an open space between adjacent edges of the second sheaths for different corners, said second sheaths each having a flange at each end, a second winding supported. on the portions of said second sheaths between. their flanges and surrounding said core, end plates of insulation enclosing said sheaths and said windings, and an enclosing casing of insulating material surrounding both of said windings, said sheaths, said partition and said core.

5. A transformer comprising a cylindrical magnetic core having a cross-section with corners, a sheath of inorganic insulation fitting said corners and leaving an open space between adjacent edges of the sheaths for different corners, said sheaths having a flange at each end, a winding supported by the portion of said sheaths between their flanges and surrounding said core, a second sheath of inorganic insulation material superposed above each first-mentioned sheath and leaving an open space between adjacent edges of difierent second sheaths, said second sheaths each having a flange at each end, and a second winding supported on the portion of said second sheaths which intervenes between the flanges thereof.

6. A transformer comprising a cylindrical magnetic core having a cross-section with corners, a sheath of inorganic insulation fitting said corners and leaving an open space between adjacent edges of the sheaths for different corners, a winding support on said sheaths and surrounding said core, a second sheath of inorganic insulation superposed above each first-mentioned sheath and leaving an open space between adjacent edges of difierent second sheaths, and a second winding supported on said second sheaths and surrounding said core.

7. A transformer comprising a cylindrical magnetic core having a cross-section with corners, a sheath of inorganic insulation fitting said corners and leaving an open space between adjacent edges of the sheaths for different corners, a winding supported on said sheaths and surrounding said core, a partition of insulating material oncircling said winding, a second sheath of inorganic insulation superposed above each firstmentioned sheath and leaving an open space between adjacent edges of difierent second sheaths, a second winding supported on said second sheaths and surrounding said core, end plates of insulating material enclosing said sheaths, said windings and said partition, and an enclosing wall of insulation surrounding the aforesaid structure.

8. A transformer comprising a cylindrical magnetic core having a cross-section with corners, a sheath of inorganic insulation fitting said corners and leaving an open space between adjacent edges of the sheaths for difierent corners, said sheaths having a flange at each end, a winding supported by the portion of said sheaths between their flanges and surrounding said core, a partition of insulating material encircling said winding, a second sheath of inorganic insulation material superposed above each first-mentioned sheath and leaving an open space between adjacent edges of different second sheaths, said second sheaths each having a flange at each end, a second winding supported on the portion of said second sheaths which intervenes between the flanges thereof, and end plates of insulating material enclosing said sheaths and said windings.

9. In combination with a pair of electrical discharge tubes to be operated in a pushpull connection, a transformer primary comprising a pair of coaxial pancake coils spaced apart by a substantial distance in an axial direction, the radially-inward ends of said pancake coils being connected together to the plate-voltage supply for said tubes, the two radially-outward ends of said coils being respectively connected to the anodes of said pushpull tubes, a secondary winding po-' sitioned relative to said pancake coils so that all portions of said winding are separated from each radially-outward end of said pancake coils by a substantial distance, said tubes and said pancake coils being so placed on a common sup port that the leads from said radially-outward terminals of said pancake coils to the respective anodes of said tubes are short and direct.

10. In combination with a pair of electrical discharge tubes to be operated in a pushpull connection, a transformer primary comprising a pair of coaxial pancake coils spaced apart by a substantial distance in an axial direction, the radially-inward ends of said pancake coils being connected together to the plate-voltage supply for said tubes, the two radially-outward ends of said coils being respectively connected to the anodes of said pushpull tubes, a secondary winding for said transformer, a portion of which comprises a third pancake coil coaxial with said two pancake coils and positioned substantially midway between them.

REUBEN LEE. 

